Strip mining is a common method of mining coal or excavating large areas of land. Therefore, strip mining operations involve the use of extremely heavy and rugged digging equipment capable of moving millions of pounds of earth. Typically, drag lines manipulate the position of a "bucket" used to dislodge, hold, and remove the soil from the site. A bucket can be virtually any size. For example, a bucket can be 35 feet long, 20 feet wide and 15 feet in depth, and weigh approximately 100,000 pounds. One end of the bucket is open with a toothed lower lip. The bucket is attached to a boom by a number of wire ropes and chains. These ropes and chains can bear the weight of the load in the bucket as well as control the orientation of the bucket.
One method of strip mining, known as "chopping," tips the bucket vertically with the toothed lip downward. The bucket is lifted several hundred feet above the ground and then allowed to free fall into the ground. The weight of the falling bucket dislodges a large quantity of soil which is then contained within the bucket. The bucket is then lifted and tilted to a horizontal orientation to prevent the soil from falling out of the bucket. The bucket is next moved to another location and tilted vertically. A spreader bar is an essential component in the dumping procedure. The spreader bar "spreads " the hoist chains away from the sides of the bucket. It is comprised of a bar that is interconnected to the hoist chains. If the spreader bar fails, the chains tend to bind against the sides of the bucket, inhibiting its ability to dump a load.
The spreader bar tends to fail in two ways. First the spreader bar can bend in the center, shortening its effective length. This is also known as arching. Second, the spreader car can simply break. In either case, the hoist chains contact the sides of the bucket inhibiting its function and even possibly damaging the bucket. Failure is caused by the cyclical loading, i.e. vibration, experienced by the spreader bar during use.
Prior art spreader bars are rigid. The central portion and ends act as one piece. Any jarring experienced by the ends during chopping are transmitted to the central portion as well. A need exists for a spreader bar that diminished the amount of force transmitted to the central portion.
The prior art spreader bars are one piece bars which are essentially straight with opposite end portions adapted for hook-up to the excavator hoist chains and chains extending downwardly to the bucket of a drag line, for example. Each opposite end of a conventional spreader bar has upper and lower pin openings which receive a pin that engages the end of the spreader bar to a lifting chain on a hoist. The lower pin opening is for the lower pin which engages a shorter chain which is connected to a pivot point on the bucket behind its center of gravity so that when the bucket is hoisted by the upper chains, the bucket will tilt into a vertical orientation.
The spreader bars are used on massive pieces of equipment which are used in the heaviest earth moving operations. For an exemplary 90 cubic yard bucket, a spreader bar may be approximately 20 inches in diameter, 18 feet long and weight approximately 20,000 pounds. When the bucket is dropped in the "chopping" operation, this huge spreader bar falls against the closed back end of the bucket as the bucket strikes the earth. Repeated use of this procedure causes the spreader bars to fail no matter what material they are made of.
Failure occurs primarily by two modes. In the first mode, the spreader bar gradually becomes arched to the point where the chains which extend downwardly from the spreader bar come in contact with the sides of the bucket and interfere with its ability to tilt and dump. Binding of these chains necessitates replacement of the spreader bar. The second mode of failure is by fracture of the spreader bar because of repeated blows. Failure of the conventional spreader bars usually occurs within three months of continuous use. It is a major operation to replace them because of the massive size of the equipment which typically may involve five or six hours of downtime on a machine that may cost $7,000 an hour to operate. Because the conventional spreader bar is in one piece and has four pine, two at each end, there are four pins to remove in order to disconnect it from the chains which may have massive links 24 inches long. The whole bar must be replaced. Therefore, a need exists for an improved spreader bar.